For consumer electronic products, digital sound effect is an important function. FIG. 1 shows the functional block diagram of a traditional multi-track speech synthesizer utilizing digital to analog converter (DAC). A DAC speech synthesizer 100 comprises three basic units, volume control unit 101, signal transform unit 102 and drive unit 103, and a plurality of signal transform units 102 can be connected directly to form a multi-track speech synthesizer.
Two volume control units 101 accept control signals Vctrl1 and Vctrl2 respectively and produce control biases Vbias1 and Vbias2. Two signal transform units 102 accept control biases Vbias1 and Vbias2 and pulse code modulation (PCM) signals PCM1 and PCM2 respectively and transform them to analog speech signal Ivo1 and Ivo2. The drive unit 103 receives the current from the directly coupled analog speech signals Ivo1 and Ivo2 and amplifies the coupled current Ivo to drive the speaker 104. FIG. 2A is the waveform of a 7-bits sinusoidal PCM signal, and FIG. 2B is the waveform of the analog speech signal Ivo after the PCM signal shown in FIG. 2A is processed by the signal transform unit 102 shown is FIG. 1. Assuming that the zero point of each analog speech signal Ivo is 1.5 mA, the direct current component increases due to the accumulation resulted from the directly coupled signals, which increases the power consumption. For applications of portable electronic products whose power supply is battery, such large power consumption should be avoided. Moreover, to prevent the transistor 105 within the drive unit 103 from saturated to result in a speech distortion, a bypass resistor 106 is inserted thereof, which further results in the speech distortion more seriously.